charging problem

["Paul C. Waterloo" <74543.407@COMPUSERVE.COM>]

JOS WROTE
Noticed, for the first time yesterday, that after starting the engine (89 100Q)
the tach has to exceed 3,000 rpm before she'll start to charge (below that rpm
the needle sits a hair south of 12v, and the warning lights are on). Funny
thing is, as soon as it starts to charge, there's a MOAN from the alternator.

What gives here?  I replaced the regulator/brushes just last spring, and had
the armature refaced at that time.  Has anyone else experienced similar beha-
vior?  Must I sacrifice a chicken on the hood of the car?

Sure would like a clue about what to look for, cause I had a long trip planned
for this coming weekend (...isn't that the way it usually happens?)

TIA

jos
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REPLY (from an old post I put out a couple of months ago)

Now for my diagnosis of your problem.  You might have a bad diode in your
alternator.  Therefore, it will have reduced output over its entire range of
operation, and until the engine gets above a certain RPM after starting, it
won't have any output.  This RPM level, if the alternator has a blown diode,
would be anywhere from 2000 to 3500 RPM.

To check if this is the problem, attach a voltmeter to the battery jump
connection under the hood with the car off.  It will read battery voltage.
Somewhere from 11.5 to 12.8 volts.

Then start the car, but do not rev the engine, only let it idle.  Note the
voltage.  If the alternator is working properly, it should be about 14 volts.
Before reeving the engine, turn on the fan on high and the headlights.  Voltage
should still be 13 volts or higher.  Then rev the engine and watch battery
voltage, if you have an alternator with a blown diode, at this time the voltage
will increase.  At 3000 RPM it should be just about 14 volts even with the
lights and fan blower on. Yours might be lower due to a blown diode.

If you find that it is the alternator, take it out and bring it to an automotive
electrical shop, they'll fix it in a day for 50-60 dollars.  I already replaced
mine with a rebuilt one ($365) that failed a year later, and then had it fixed
for $60.

Hope this helps.  Paul Waterloo